Predictors of tick-associated rash illness (TARI) among tick bite patients: Roles of exposure history and tick biological characteristics.

BACKGROUND

Tick-borne diseases are an important public health concern worldwide. In Japan, tick-associated rash illness (TARI) has been frequently reported following bites from Amblyomma testudinarium. However, the epidemiological characteristics and risk factors associated with TARI are not well understood. This study aimed to characterize tick bite cases and identify predictors associated with the development of TARI.

METHODS

We analyzed 523 tick bite cases reported by medical clinics in Hyogo and Shizuoka Prefectures, Japan, between 2019 and 2024. Data on demographics, clinical manifestations, tick bite history, and tick characteristics were collected. Although tick bite cases associated with different tick species were recorded, statistical analyses were restricted to A. testudinarium-associated cases because most TARI cases occurred in this group. Associations with TARI were evaluated using comparative analyses and multivariable logistic regression.

RESULTS

Most tick bite cases were associated with A. testudinarium (84.5%). The average age of individuals was 54 years. Increasing age was independently associated with higher TARI risk (adjusted odds ratio [aOR] = 1.06 per year; 95% confidence interval [CI] 1.04-1.10). A strong exposure-response relationship was observed between prior tick bites and TARI risk: patients reporting three or more previous tick bites had 7.0-fold higher odds of developing TARI (95% CI 3.04-16.80) compared with first-time exposures. Tick biological characteristics were also significant predictors: bites from larval ticks were associated with 8.52-fold higher odds of TARI than bites from nymphal ticks (95% CI 1.48-58.10), and moderately engorged ticks carried 3.14-fold higher odds compared with unfed ticks (95% CI 1.23-8.71). Season was also associated with TARI occurrence, with significantly lower odds observed in summer and autumn compared with spring.

CONCLUSION

History of repeated tick bites and biological characteristics of the removed ticks seem to be major factors associated with TARI risk, supporting the hypothesis that TARI may represent an immunologically mediated hypersensitivity reaction. Given the broader distribution of A. testudinarium in Southeast Asia, clinical awareness and detailed exposure assessment are needed beyond Japan to improve diagnosis and guide public health strategies for at-risk populations across the region.